iOS获取验证码倒计时效果

/**重新发送短信的计时*/
-(void)fireTimer{
 __block int timeout=180; //倒计时时间
 dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

 dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,queue);

 dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); //每秒执行

 dispatch_source_set_event_handler(_timer, ^{
  if(timeout<=0){ //倒计时结束，关闭
   dispatch_source_cancel(_timer);
   dispatch_async(dispatch_get_main_queue(), ^{
    //设置界面的按钮显示 根据自己需求设置
    [_changeCodeButton setTitle:@"获取验证码" forState:UIControlStateNormal];
    _changeCodeButton.userInteractionEnabled = YES;
   });
  }else{
   int seconds = timeout ;//% 60;
   NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];
   dispatch_async(dispatch_get_main_queue(), ^{
    //设置界面的按钮显示 根据自己需求设置
    //NSLog(@"____%@",strTime);
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:1];
    _changeCodeButton.titleLabel.font = [UIFont systemFontOfSize:14];
    [_changeCodeButton setTitle:[NSString stringWithFormat:@"(%@s)后重新获取",strTime] forState:UIControlStateNormal];
    [UIView commitAnimations];
    _changeCodeButton.userInteractionEnabled = NO;
   });
   timeout--;
  }
 });
 dispatch_resume(_timer);
}